GENERAL GENETICS AYESHA MASRUR KHAN SPRING 2013

G ENE INTERACTION What is gene interaction? In gene interaction, genes at different loci contribute to the determination of a single phenotypic characteristic. With gene interaction, the products of genes at different loci combine to produce new phenotypes that are not predictable from the single-locus effects alone. 2

C APSICUM ANNUUM F RUIT COLOR IN THE PEPPER C APSICUM ANNUUM This plant produces peppers in one of four colors: red, brown, yellow, or green. 3  In peppers, a dominant allele R at the first locus produces a red pigment; the recessive allele r at this locus produces no red pigment.  A dominant allele C at the second locus causes decomposition of the green pigment chlorophyll; the recessive allele c allows chlorophyll to persist.

 The genes at the two loci then interact to produce the colors seen in F2 peppers 4

C OMB SHAPE IN CHICKENS  Genes at two loci ( R, r and P, p) interact to determine the four types of combs shown: R_P_ 1. Walnut comb R_P_ R_pp 2. Rose comb R_pp rrP_ 3. Pea comb rrP_ rrpp 4. Single comb rrpp 5

G ENE I NTERACTION WITH E PISTASIS epistatic gene hypostatic gene  Epistasis is the masking of the expression of one gene by another gene at a different locus. The epistatic gene does the masking; the hypostatic gene is masked.  Epistatic genes can be dominant or recessive. Similar to dominance, except that dominance entails the masking of genes at the same locus (allelic genes). 6

R ECESSIVE EPISTASIS R ECESSIVE EPISTASIS C OAT COLOR IN L ABRADOR RETRIEVERS : BLACK, BROWN, OR YELLOW One locus determines the type of pigment produced by the skin cells: a dominant allele B codes for black pigment, whereas a recessive allele b codes for brown pigment. Another locus affects the deposition of the pigment in the shaft of the hair; allele E allows dark pigment (black or brown) to be deposited, whereas a recessive allele e prevents the deposition of dark pigment, causing the hair to be yellow. 7

R ECESSIVE EPISTASIS ( CONTD.) Coat color in Labrador retrievers: black, brown, or yellow Genotype Phenotype B_ E_ black bbE_ brown (frequently called chocolate) B_ee yellow bbee yellow Allele e is epistatic to B and b, because e masks the expression of the alleles for black and brown pigments, and alleles B and b are hypostatic to e. In this case, e is a recessive epistatic allele, because two copies of e must be present to mask the black and brown pigments. 8

D OMINANT EPISTASIS D OMINANT EPISTASIS FRUIT COLOR IN SUMMER SQUASH : YELLOW, WHITE, OR GREEN Genes affect a series of steps in a biochemical pathway. Plants with the genotype ww produce enzyme I and may be green or yellow, depending on whether enzyme II is present. When allele Y is present at a second locus, enzyme II is produced and compound B is converted into compound C, producing a yellow fruit. When two copies of y, which does not encode a functional form of enzyme II, are present, squash remain green. The presence of W at the first locus inhibits the conversion of compound A into compound B; plants with genotype W_ do not make compound B and their fruit remains white, regardless of which alleles are present at the second locus. 9

Yellow pigment in summer squash is produced in a two-step pathway 10

D UPLICATE RECESSIVE EPISTASIS Example: In sweet pea, C/- is colored flower and c/c is white. If colored, P/- is purple and p/p is white. c/c P/- is same as C/- p/p Result is F2 ratio of 9:7 11

S EX -I NFLUENCED C HARACTERISTICS  Sex-influenced characteristics are traits encoded by autosomal genes that are more readily expressed in one sex.  The trait has higher penetrance in one of the sexes. 12

S EX -I NFLUENCED C HARACTERISTICS ( CONTD ) B b The presence of a beard on some goats is determined by an autosomal gene (B b ) that is dominant in males and recessive in females. B b B b B b B + B + B +  In males, a single allele is required for the expression of this trait: both the homozygote (B b B b ) and the heterozygote (B b B + ) have beards, whereas the B + B + male is beardless. B b B b B b B + B + B +  In females require two alleles in order for this trait to be expressed: the homozygote B b B b has a beard, whereas the heterozygote (B b B + )and the other homozygote (B + B + ) are beardless. The key to understanding the expression of the bearded gene is to look at the heterozygote. In males (for which the presence of a beard is dominant), the heterozygous genotype produces a beard but, in females (for which the presence of a beard is recessive and its absence is dominant), the heterozygous genotype produces a goat without a beard. 13

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S EX -I NFLUENCED C HARACTERISTICS ( CONTD ) Pattern baldness Autosomal character Dominant in males and recessive in females Men require only a single allele for baldness to become bald, whereas women require two alleles for baldness. Weakly expressed in women; general thinning of the hair all over the scalp Men frequently lose all the hair on the top of the head; allele for pattern baldness is clearly enhanced by the presence of male sex hormones. Genotyp e Phenotype in Females Phenotype in Males BBbald Bbnonbaldbald bbnonbald 16

S EX - LIMITED C HARACTERISTICS  Sex-limited characteristics are encoded by autosomal genes whose expression is limited to one sex.  The trait has zero penetrance in the other sex. 17

S EX - LIMITED C HARACTERISTICS ( CONTD ) In domestic chicken, some males display a pattern called cock feathering; other display a pattern called hen feathering. Cock feathering is an autosomal recessive trait that is sex limited to males. Genotype Male phenotype Female phenotype HH hen feathering hen feathering Hh hen feathering hen feathering hh cock feathering hen feathering The pattern of hen-feathering depends on the production of sex hormones. 18